Abstract: IMPACTS OF VOLUME FRACTION ON CRITICAL SHEAR FORCE FOR DILATANT COLLOIDS AND OF SOLVENT POLARITY ON DILATANCY. Sam Wood and Julian D’Rozario. The purpose of this experiment was to determine the relationship between volume fraction and critical shear force of dilatant colloids of cornstarch and water and to determine the influence of the polarity of the solvent on the dilatancy of cornstarch-based mixtures. In the lab, three mixtures of cornstarch and water, with volume fractions 3:2, 2:1, and 11:5, and three mixtures of cornstarch and heptane (C7H16), with volume fractions 1:1, 2:1, and 3:1, were prepared. The force required to cause the cornstarch and water mixtures to exhibit solid-like behavior was determined using a force plate and weights of varying mass. Force data were also collected for solid ice, liquid water, and a gel-like mixture of water and gelatin to establish a point of reference for the behavior exhibited by the cornstarch and water colloids. The data collected for the cornstarch and water colloids support the hypothesis that as volume fraction increases, critical shear force decreases. As expected, the critical shear force for the 3:2 mixture, either somewhere between 19.5 and 42.2 N or higher than 42.2 N, was the highest; the critical shear force for the 11:5 mixture, between 0 and 1.6 N, was the lowest; and the critical shear force for the 2:1 mixture, between 14.1 and 16.2 N, fell in between. These results, along with the observations from the experiment, support the model of hydroclustering to explain the non-Newtonian behavior of dilatant materials. Observations of the cornstarch and heptane mixtures, which did not demonstrate dilatant behavior, suggest that the electrostatic-attraction model may be more viable than the polymer-tangling model for jamming within cornstarch-based colloids. However, due to the large size differences between water and heptane, the results do not necessarily distinguish between the two models. Unexpectedly, the results of the experiment suggest that a given volume fraction may not have a definite, determinable critical shear force. Further work must be performed to distinguish between the electrostatic-attraction and the polymer-tangling models and to verify the finding that critical shear force is a range rather than an exact value. Keywords: dilatancy, colloid, Newtonian fluid, rheology, deformation, critical shear force, hydrocluster, volume fraction, heptane.
Lab Apparatus:
Summary Graphic: The numbers on the bars denote the force exerted on the substance during the trial.
Smith, M. I., Besseling, R., Cates, M. E., & Bertola, V. (2010). Dilatancy in the flow and fracture of stretched colloidal suspensions. Nature Communications, 1(114), 1-5. doi:10.1038/ncomms1119
Wagner, N. J., & Brady, J. F. (2009). Shear thickening in colloidal dispersions. Physics Today, 62(10), 27-32.
IMPACTS OF VOLUME FRACTION ON CRITICAL SHEAR FORCE FOR DILATANT COLLOIDS AND OF SOLVENT POLARITY ON DILATANCY. Sam Wood and Julian D’Rozario. The purpose of this experiment was to determine the relationship between volume fraction and critical shear force of dilatant colloids of cornstarch and water and to determine the influence of the polarity of the solvent on the dilatancy of cornstarch-based mixtures. In the lab, three mixtures of cornstarch and water, with volume fractions 3:2, 2:1, and 11:5, and three mixtures of cornstarch and heptane (C7H16), with volume fractions 1:1, 2:1, and 3:1, were prepared. The force required to cause the cornstarch and water mixtures to exhibit solid-like behavior was determined using a force plate and weights of varying mass. Force data were also collected for solid ice, liquid water, and a gel-like mixture of water and gelatin to establish a point of reference for the behavior exhibited by the cornstarch and water colloids. The data collected for the cornstarch and water colloids support the hypothesis that as volume fraction increases, critical shear force decreases. As expected, the critical shear force for the 3:2 mixture, either somewhere between 19.5 and 42.2 N or higher than 42.2 N, was the highest; the critical shear force for the 11:5 mixture, between 0 and 1.6 N, was the lowest; and the critical shear force for the 2:1 mixture, between 14.1 and 16.2 N, fell in between. These results, along with the observations from the experiment, support the model of hydroclustering to explain the non-Newtonian behavior of dilatant materials. Observations of the cornstarch and heptane mixtures, which did not demonstrate dilatant behavior, suggest that the electrostatic-attraction model may be more viable than the polymer-tangling model for jamming within cornstarch-based colloids. However, due to the large size differences between water and heptane, the results do not necessarily distinguish between the two models. Unexpectedly, the results of the experiment suggest that a given volume fraction may not have a definite, determinable critical shear force. Further work must be performed to distinguish between the electrostatic-attraction and the polymer-tangling models and to verify the finding that critical shear force is a range rather than an exact value. Keywords: dilatancy, colloid, Newtonian fluid, rheology, deformation, critical shear force, hydrocluster, volume fraction, heptane.
Lab Apparatus:
Summary Graphic:
The numbers on the bars denote the force exerted on the substance during the trial.
Recording:
References:
Efremov, I. F. (1982). The dilatancy of colloidal structures and polymer solutions. Retrieved from http://iopscience.iop.org/0036-021X/51/2/R05
Botsau, C., & McDaniel, K. (2010). Oobleck: lab activity. Retrieved from http://www.asminternational.org/content/CityOfMaterials/pdfs/12-%20Oobleck%205.09.10.pdf
Smith, M. I., Besseling, R., Cates, M. E., & Bertola, V. (2010). Dilatancy in the flow and fracture of stretched colloidal suspensions. Nature Communications, 1(114), 1-5. doi:10.1038/ncomms1119
Wagner, N. J., & Brady, J. F. (2009). Shear thickening in colloidal dispersions. Physics Today, 62(10), 27-32.